There exists a need for an optical material capable of both light diffusion and concentration of the diffused light to provide a brighter, though narrower field of view. Such a device would be useful, for example, in a backlighting arrangement for a flat-panel display such as a liquid-crystal (LCD) display. Light diffusers are now common in such backlighting configurations, where they convert light from lamp segments or discrete sources into a more uniform glow across the surface of the display. By their very nature, however, to obscure the light source(s) and cause their shape to be less visually apparent, such diffusers panels redirect incident radiation into multiple output rays which assist in filling the gaps otherwise visible between the discrete sources. This results in a more or less uniform illumination of the display screen, but over a relatively wide range of viewing angles.
If it is known that a narrower range of viewing angles will be used, however, it may be advantageous to concentrate the diffused light into a narrower range of output angles. Representative applications include aircraft and automotive instrumentation, wherein the relative position between the display and the viewer may be substantially fixed. Even in computer display and television viewing environments, however, it may be desirable to reduce or eliminate extremely wide viewing angles in favor of concentrating more light directly in front of the screen. Indeed, in portable computer applications, concentrating light most toward the operator provides a certain level of security by preventing others from being able to view what might be sensitive data.
Concentration of diffused light may be accomplished with several techniques, including the addition of a second, focusing element disposed between the diffuser and the back side of the display panel. Brightness enhancement film (BEF) available from 3M Optical Systems of St. Paul, Minn., for example, provides a light-exiting surface having a multiplicity of prisms, each performing a double internal reflection wherein about half of the incoming rays are reflected back and recycled unless they can be output within a narrower exit cone, thereby providing a brightening effect for a particular set of viewing angles. Such BEF film however, does not provide a diffusing function, but instead, must necessarily be used with a separate diffusing member if such an additional function is desired.
As a modern alternative, volume-phase holograms may be recorded not only to scatter incident light to provide a diffusion function, but also to scatter such light in a directional manner in accordance a preferred range of viewing angles. U.S. Pat. No. 5,418,631 teaches the use of a volume hologram for use in an edge-lit application, and U.S. Pat. No. 5,471,327 teaches a hologram recorded to directionally diffuse light from an array of linear lamp segments.
One problem with the use of volume-phase material is that it is relatively expensive as compared to a surface-relief hologram, and highly controlled coherent laser exposure is required during the recording process. Moreover, in light-concentrating applications holographic surface-relief structure often out-perform volume-phase counterparts. The need remains, therefore, for a surface-relief type of material which provides both a light-diffusing function and light-concentration function which is highly directional yet economical to manufacture.